This invention relates to a continuous process for producing a fully dense ceramic material, in particular an aluminum oxide-titanium carbide or carbonitride composite material suitable for use in cutting tools and wear resistant structural materials.
U.S. Pat. No. 4,490,319 to M. Lee et al. discloses a rapid rate sintering process for ceramics and composites including Al.sub.2 O.sub.3 --TiC. The maximum density achieved by rapid sintering is 98% of the theoretical density. High-purity helium is used as the sintering atmosphere in a molybdenum-element furnace. The examples do not include the use of TiO.sub.2 as a sintering aid. The process is a two-step process involving sintering followed by hot isostatic pressing (a sinter-HIP process). M. Lee et al. ("Rapid Rate Sintering of Al.sub.2 O.sub.3 --TiC Composites for Cutting Tool Applications", Advanced Ceramics Materials 3 [1], 38-44 (1988)) report the findings disclosed in above-referenced U.S. Pat. No. 4,490,319. In addition, they describe the use of a 100% CO or CO-helium (at partial pressures ranging from 1:40 to 1:1) sintering atmosphere, which their data show to be detrimental to rapid sintering of Al.sub.2 O.sub.3 --TiC.
U.S. Pat. No. 4,063,908 to K. Ogawa et al. discloses the use of TiO.sub.2 densification aid for hot pressed Al.sub.2 O.sub.3 --TiC. However, the TiO.sub.2 densification aid is used for hot pressing only. Harmer et al. (Proc. 4th International Mtg. on Modern Ceramics Technologies, St. Vincent, Italy, May 1979, ed. P. Vincenzini, Elsevier Sci. Publ. Co., Amsterdam, 1980, 155-162) report fast firing of alumina ceramics doped with MgO or TiO.sub.2 in a zone sintering furnace at 1850.degree. C. for 2-15 min. in air or oxygen.
U.S. Pat. No. 4,797,238 to W. H. Rhodes et al. discloses a rapid sintering process for improving translucence in alumina lamp tubes. The heating rate is at least 0.1.degree. C./sec. and less than the rate at which the green body experiences thermal shock. The process uses a flowing gas mixture of nitrogen and at least about 2.5 v/o (volume percent) hydrogen to produce a translucent polycrystalline alumina lamp tube in a belt furnace. U.S. Pat. No. 4,762,655 to W. H. Rhodes et al. discloses the use of a sintering atmosphere of nitrogen and an amount of hydrogen of at least 2.5 v/o and less than 75 v/o to produce a translucent polycrystalline alumina lamp tube in a static furnace. In U.S. Pat. No. 4,948,538 to G. C. Wei et al., the dew point of a N.sub.2 -8 v/o H.sub.2 sintering atmosphere is controlled while producing a translucent alumina lamp tube in a belt furnace or a static furnace.
R. A. Cutler et al. ("Pressureless-sintered Al.sub.2 O.sub.3 --TiC Composites", Mat. Sci. and Eng., A105/106, 183-192 (1988)) disclose sintering of Al.sub.2 O.sub.3 containing 26.3 w/o (weight percent) TiC and 3.7 w/o TiH.sub.2 at 1840.degree. C. in nitrogen to 4.22 g/cc density or 97.5of the theoretical density, at a moderate heating rate, 40.degree.-50.degree. C./min., in a graphite furnace. T. Ishigaki et al. ("Pressureless Sintering of TiC--Al.sub.2 O.sub.3 Composites", J. Mat. Sci. Letters 8, 678-680 (1989)) disclose the addition of 0.5 w/o MgO to Al.sub.2 O.sub.3 -25 v/o TiC resulting in a sintered density of 99% of theoretical in uniaxially hot pressed samples.
Y. Kim et al. ("Pressureless Sintering of Alumina-TiC Composites", J. Am. Cer. Soc. 72 [8], 1333-37 (1989)) use a setter powder of alumina, TiC, carbon, and combinations thereof to achieve a sintered density of 96.7% of theoretical for Al.sub.2 O.sub.3 containing 30 w/o TiC and doped with 0.1 w/o MgO. M. Furukawa et al. ("Fracture Toughness of the System Al.sub.2 O.sub.3 --TiC Ceramics", Nippon Tungsten Review 18, 16-22 (1985)) disclose that the fracture toughness of alumina composites increases with increasing TiC content up to a maximum at 60 w/o TiC, and then decreases with increasing TiC content.
R. P. Wahi et al. ("Fracture Behavior of Composites based on Al.sub.2 O.sub.3 --TiC", M. Mat. Sci. 15, 875-885 (1980)) observe that the fracture toughness of alumina composites in machined-notch tests increases with increasing TiC content and appears to reach a plateau at about 20 w/o TiC. Japanese patent 63-134644 discloses the use of Al.sub.2 O.sub.3 --TiC setter powder to drive the partial pressure of CO to 10.sup.-4 -10.sup.-3 atm during sintering of Al.sub.2 O.sub.3 --TiC. However, no use of TiO.sub.2 sintering aid or rapid rate sintering was mentioned.
GTE Valenite Corporation produces Al.sub.2 O.sub.3 cutting tools from a starting composition of Al.sub.2 O.sub.3, TiC, MgO, and ZrO.sub.2.